Liquid crystal display devices have widely been utilized for tabletop calculators, word processors, television sets, and the likes, including watches. As their applications became upscale, however, liquid crystal compositions having more appropriate physical property values became necessary. As the result, liquid crystal compositions having such a low threshold voltage that devices were capable of being driven at a low voltage, and liquid crystal compositions having such a high voltage holding ratio that the reliability of devices was increased were developed.
Threshold voltage (Vth) is expressed by the following equation (H. J. Deuling et al., Mol. Cryst. Liq. Cryst., 27 (1975) 81): EQU Vth=.pi.(K/.di-elect cons.0.DELTA..di-elect cons.).sup.1/2
wherein K denotes an elastic constant, .di-elect cons.0: a dielectric constant in vacuum, and .DELTA..di-elect cons.: a dielectric anisotropy value. From this equation, it can be understood that in order to lower threshold voltage, it is sufficient to increase the dielectric anisotropy or decrease the elastic constant. Generally, dielectric anisotropy is more often controlled than elastic constant, since many liquid crystal compounds having a high dielectric anisotropy are known.
Besides, threshold voltage depends also on the pretilt angle of liquid crystal compositions induced in a device. When both dielectric anisotropy and elastic constant are constant, the larger the pretilt angle is, the lower the threshold voltage tends to be. Compounds of the formula (10) described in Laid-open Japanese Patent Publication No. Hei 2-501311 induce a comparatively large pretilt angle. ##STR2##
in the structural formula described above, R represents an alkyl group.
Liquid crystal compositions comprising a compound of the formula (10) are promising since they are low in dependency of voltage holding ratio on temperature, induce a comparatively large pretilt angle, and have a high reliability. However, they are not suitable for the applications wherein devices are driven at such a low voltage of 2.5 V, because their dielectric anisotropy values are such a medium extent as about 7 since contribution of trifluoromethoxy group to dielectric anisotropy is small.
On the other hand, it is known that liquid crystal compositions comprising a fluorine type liquid crystalline compound are high in voltage holding ratio and low in their dependency on temperature. ##STR3##
in the structural formulas described above, R represents an alkyl group.
Compounds having a plural number of fluorine atom at a terminal of the molecule of the formula (11) (DE-4027840A1), and formulas (12-1) and (12-2) (Laid-open Japanese Patent Publication No. Hei 2-233626) have a comparatively high dielectric anisotropy. However, when compounds of the formula (11) are used as component of liquid crystal compositions, the extent to which threshold voltage of the compositions is lowered is small since the compounds have a large elastic constant whereas trifluoromethyl group in the compounds contributes to a high dielectric anisotropy. Also, they induce a low pretilt angle. Accordingly, the compounds can not be said to be suitable for the applications at a low voltage. When compounds of the formulas (12-1) are used, the compositions exhibit a comparatively high voltage holding ratio and are small in dependency of voltage holding ratio on temperature. However, their dielectric anisotropy is lower than the case where a compound of the formula (11) is used. Whereas compounds of the formula (12-2) have a high dielectric anisotropy due to the contribution by 3,4,5-trifulorophenyl group and ester bonding group, and are larger in contribution to the pretilt angle in liquid crystal compositions than compounds of the formula (11), the compounds are large in dependency of voltage holding ratio on temperature. Thus, applications of compounds of the formula (12-2) are limited. Accordingly, liquid crystalline compounds from which liquid crystal compositions having a low threshold voltage, exhibiting a high voltage holding ratio, and being small in dependency of voltage holding ratio on temperature can be produced have been desired.